1. Field of the Invention
The present invention relates generally to automated systems for metering and delivering liquids. More particularly, the present invention relates to automated spray painting systems used for applying electrostatic paints.
2. Description of Related Art
Many different industries utilize automated spray painting systems in order to provide a final protective coating to mass-produced articles. Such automated spray painting systems are widely used in the automobile industry due to the large number of cars produced and the requirement that a high-quality paint finish be provided to control corrosion and provide an attractive visual appearance.
Electrostatic paints are widely used in automated spray painting systems because of the improved coating coverage achieved with such paints and the reduction in waste. The object to be painted, such as an automobile, is electrostatically charged so that the electrostatically charged paint will be attracted to the automobile. It is important that the electrical charge present in the paint during spray application be within certain desired limits. Otherwise, incomplete coverage or excessive overspray will occur. Thus, the efficiency of an electrostatic paint spraying process depends on the accurate control of the electrical conductivity of the paint.
Typically, the conductivity of the paint is monitored by placing a hand-held probe into the paint reservoir which is designed to measure the direct current (DC) conductivity of the paint. Although such a monitoring system is suitable in some situations, it does have certain drawbacks in that DC measurements are subject to concentration polarization effects in which polarized species and thus the electric field become concentrated at the electrodes, lower the ion current to the electrode, and produce inaccurate measurements. In addition, such DC conductivity measurement systems do not provide accurate measurement of the paint conductivity at the most critical location, i.e., where the paint enters the spray gun for atomization. It is possible that the conductivity of the paint may change as it travels through the automated spray system so that the conductivity of the paint as it reaches the spray gun is substantially different from the conductivity measured in the paint reservoir. It would be desirable to have an automated electrostatic paint spray system in which the electrical conductivity of the paint is accurately monitored close to the point of delivery of the paint as a spray.
In addition most automated paint systems are designed to spray more than one color of paint. Typically, solvent is pumped through the system between paint changes. The amount of time and solvent which is necessary to flush paint from the system and begin spraying with the different paint is determined empirically. As a result, solvent and paint are wasted because excess solvent and/or paint is typically pumped through the system to ensure complete system cleaning and changeover of paint. It would be desirable to provide an automated paint system in which paint changeover can be continually monitored and accomplished with minimum waste of solvent and paint.